Hydrobiologia 434: 11–16, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 11 Species interactions between estuarine detritivores: inhibition or facilitation? Charles T. Chong 1 , Scott T. Larned 2 , Alan P. Covich 3 & Robert A. Kinzie III 4,∗ 1 Zoology Department, University of Hawai‘i, Honolulu, HI 96822, U.S.A. 2 Coastal Ecology Branch, United States Environmental Protection Agency, Newport, OR 97365, U.S.A. 3 Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, CO 80523, U.S.A. 4 Zoology Department and Hawai‘i Institute of Marine Biology, University of Hawai‘i, Honolulu, HI 96822, U.S.A. E-mail: rkinzie@zoology.hawaii.edu ( ∗ Author for correspondence) Received 9 March 2000; in revised form 9 March 2000; accepted 21 April 2000 Key words: ecological redundancy, competition, detritus processing, species interactions, tropical estuaries Abstract Native Hawaiian estuarine detritivores; the prawn Macrobrachium grandimanus, and the neritid gastropod Neritina vespertina, were maintained in flow-through microcosms with conditioned leaves from two riparian tree species, Hau (Hibiscus tiliaceus) and guava (Psidium guajava). Their ability to beak down leaf detritus was determined when alone and when they were together. In single-species treatments, N. vespertina processed leaves from both trees at higher rates than M. grandimanus, but in combined treatments, facilitation occurred when the substrate consisted of Hau leaves, and interference occurred when the substrate consisted of guava leaves. From this, we conclude that whether detritivore species are functionally redundant, facilitating or inhibiting in their processing of detritus depends not only on the detritivore species, but also on the species composition of the detritus food source. Introduction Questions of species-specific functions, species inter- actions and species redundancy are of general and increasing interest to ecologists studying ecosystem processes (Jones & Lawton, 1995; Finlay et al., 1997; Ehrlich & Walker, 1998; Naeem, 1998). This in- terest stems from concerns about the stability and diversity of natural ecosystems and how such systems respond to loss (or addition) of species (Ehrlich & Ehr- lich, 1981; Robinson & Dickerson, 1987; Lawton & Brown, 1993; Bond, 1993; Lawton, 1994). While most attention has been given to terrestrial systems, the underlying ecological principles should be similar in aquatic habitats. Low species diversity, limited spatial complexity and dependence on a lim- ited range of food sources make tropical estuaries ideal for the study of questions of species interactions and ecosystem function. In estuarine ecosystems, benthic food webs are often linked to terrestrial as well as in situ production, with biotic processing of coarse particulate organic matter of terrestrial origin being a fundamental pathway of energy flow (Odum, 1980, 1984; Day et al., 1989). Rates at which coarse detritus is converted to fine particles and dissolved matter in estuarine food webs are influenced by the composi- tion of the detritus and the species composition of the macrofauna. In estuaries, there are typically a few generalist animal species, most of which carry out similar eco- system functions. There is, however, a possibility that interactions between and among these species could increase or decrease rates of detritus processing (Day et al., 1989). The term facilitation describes cases where two or more species have combined effects on the rates of detritus processing that are greater than the sum of their individual effects. The term inhibition describes cases in which rates of detritus